516 Mr. A. Willey. On the Post-Embryonic [May 19, 



Fig-. 2. — Anterior end of young larva of AmpJiioxus. From right side. Praeoral 

 pit (p.p.) and mouth are seen through. 



gl. y club-shaped gland; n, notochord. Other letters as in fig. 1. 



up to the present time there has been no gill-slit described in 

 Ascidians which possessed characters peculiar to itself, and which 

 separated it from the rest. Such a gill-slit, however, exists, and I 

 will now describe it. 



Starting with the stage in which two oval apertures are present on 

 each side (in Giona), we find that, as time goes on, these elongate very 

 considerably in the transverse direction, and eventually become 

 twisted round in a curious way at their distal ends (that is, the ends 

 towards the endostyle), and finally a small portion becomes con- 

 stricted off' from the distal end of each of the original slits (fig. 3). 



In this way, therefore, we arrive at the stage with four branchial 



